Optical resonators include two or more mirror structures that define the resonator cavity. Optical resonators can be passive cavity devices as used, for example, in tunable Fabry-Perot (FP) filters. Active cavity devices include active media inside the optical resonator, such as gain medium, nonlinear optical medium, or electro-optic medium. The most common example of an active cavity optical resonator is the laser, which contains a gain medium, such as a semiconductor or a solid-state material, inside the cavity between the mirror structures.
A reoccurring issue in optical resonator design, both in macroscopic and micro optical systems, is transverse spatial mode control. At scales associated with micro optical systems, which include single mode optical fiber, semiconductor gain media, and/or micro-opto-electro-mechanical system (MOEMS) devices, spatial mode control can dictate many system design variables.
Typically, fundamental transverse mode operation is desired in laser devices because of the optical beam spatial profile requirements for long distance beam propagation, focusing of beams into small spots, and beam coupling into single mode transmission fibers. In addition, different spatial modes of an optical resonator typically have different resonant optical frequencies, which characteristic is detrimental for active and passive cavity applications requiring spectral purity. A typical application requiring spectral purity of the resonator operation is optical spectral monitoring using tunable Fabry-Perot filters of optical signals such as wavelength-division-multiplexed (WDM) optical communication signals.
In active cavity devices, such as edge-emitting semiconductor lasers, the transverse spatial mode problem is addressed by the judicious design of the laser waveguide to ensure that it supports only a single transverse mode. In vertical cavity surface emitting lasers (VCSEL's), oxide confining layers and other aperturing techniques are used to achieve single transverse mode operation in small aperture devices. Problems begin to arise as higher output power devices are designed, however. Here there is a contention between the desire to increase modal volume and beam diameter while at the same time suppressing oscillation of the higher-order transverse modes. Gain aperturing is frequently used inside laser cavities to suppress lasing of the higher-order transverse modes.
In passive cavity devices, the transverse mode problems can be more intractable, since the design degree of freedom associated with the gain medium is not present. One solution is to incorporate single mode fiber into the design. The inclusion of fiber, however, tends to complicate increased device integration, creates fiber-coupling requirements, and does not resolve all of the spatial mode problems. For example, a detector can be responsive to the input higher-order mode even with spatial mode filtering offered by a single mode fiber; this is due to the substantial amount of power propagating in the leaky and cladding modes of the fiber.
A related solution to controlling the transverse side mode suppression ratio (SMSR) contemplates the use of intracavity apertures or spatial filters. Higher order spatial modes generally have larger mode field diameters than the fundamental TEM00 mode. As a result, apertures in the optical train can induce loss for higher order transverse modes and may be used to improve the side mode suppression. These spatial filters, however, can introduce some loss to the fundamental mode as well as to the higher order modes; they also require precise alignment.
Still another solution that addresses the spectral purity problem concerns cavity design. In a confocal Fabry-Perot cavity, where cavity length is equal to the mirror radius of curvature, all transverse modes are degenerate, i.e., all the transverse modes coexist on the same set of frequencies, or wavelengths, as the longitudinal mode frequencies or the longitudinal mode frequencies shifted by a half spectral period. MOEMS micro optical cavities typically have large free spectral ranges, or spectral periods, corresponding to small cavity lengths of only tens of micrometers, however. Therefore the confocal MOEMS micro cavity configuration would require mirrors with correspondingly small radii of curvature, i.e., tens of micrometers, which are difficult to fabricate, and have small mode sizes, which are difficult to align.
A more typical configuration for MOEMS tunable filter Fabry-Perot cavity is termed a hemispherical cavity. In such cavities, one of the reflectors is near planar and the other reflector is a spherical reflector. Such configurations are used both for passive cavity tunable filters and for vertical-cavity surface-emitting (VCSEL) lasers with an external fixed or tunable mirror. The advantage of such cavities is reduced alignment criticalities because of the general radial homogeneity of the flat reflector. In such configurations, spatial mode spectral degeneracy is not present and higher order transverse modes present a problem, for example spurious peaks are observed in the tunable filter transmission spectrum.
These problems have led to solutions that focus on minimizing the excitation of higher order modes by precise control of how light is launched into the cavity. For example, U.S. patent application Ser. No. 09/666,194, filed on 21 Sep. 2000 by Jeffrey A. Korn, and 09/747,580, filed 22 Dec. 2000 by Walid A. Atia, et al., concern, in part, alignment of a tunable filter relative to the surrounding optical train. U.S. patent application Ser. No. 09/809,667, filed on 15 Mar. 2001 by Jeffrey A. Korn, concerns mode field matching between the launch light mode and the lowest order spatial mode of the filter. Such approaches minimize excitation of higher order spatial modes and thus yield systems with high side mode suppression ratios.